The organic electroluminescence devices (organic EL devices) are spontaneous light emitting devices which utilize the principle that a fluorescent substance emits light by energy of recombination between holes injected from an anode and electrons injected from a cathode upon application of an electric field thereto.
Since C. W. Tang, et al., of Eastman Kodak Company have reported organic EL devices of a laminate type driven at a low electric voltage (C. W. Tang and S. A. Vanslyke, “Applied Physics Letters”, Vol. 51, p. 913, 1987, etc.), many studies have been intensely conducted on organic EL devices made of organic materials.
The organic EL devices reported by Tang, et al., have such a laminate structure including a light emitting layer made of tris(8-hydroxyquinolinol)aluminum and a hole transport layer made of a triphenyl diamine derivative. The laminate structure of these devices has advantages such as increased efficiency of hole injection into the light emitting layer, increased efficiency of production of excited particles (excitons) which are produced by blocking electrons injected from a cathode and recombining the electrons with holes, and confinement of the excitons produced within the light emitting layer. As the structure of such organic EL devices, there are well known a two-layer structure including a hole transporting (injecting) layer and an electron transporting and light emitting layer, a three-layer structure including a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer, etc. In these organic EL devices of a laminate type, various structures and production methods thereof have been proposed in order to enhance an efficiency of recombination between holes and electrons injected thereinto.
As the light emitting materials for the organic EL devices, there are known chelate complexes such as tris(8-quinolinolato)aluminum complexes, coumarin derivatives, tetraphenyl butadiene derivatives, bis-styryl arylene derivatives and oxadiazole derivatives. It has been reported that these light emitting materials emit blue to red light in a visible range, and it is therefore expected to realize color display devices by using these light emitting materials (for example, refer to JP 8-239655A, JP 7-138561A and JP 3-200889A, etc.).
In addition, JP 2001-207167A discloses the devices using aminoanthracene derivatives as a green light emitting material. However, the light emitting material has failed to provide devices having a long life and a high efficiency of light emission and, therefore, has been practically unusable.
In recent years, although many organic EL devices having a high luminance and a long life have been disclosed or reported, the performance thereof has been still unsatisfactory. Therefore, is has been strongly demanded to develop materials for organic EL devices having a more excellent efficiency of light emission.